Our objectives are to describe developmental changes in various aspects of tissue respiration during fetal and neonatal development and to ascertain the effects of changes in thyroid status on these parameters. The tissues under investigation are brown adipose tissue (BAT), liver, brain, kidney and heart. To date we have characterized a developmental increase in both basal and stimulated BAT cellular respiration in the fetal rabbit. Both basal and stimulated respiration increase several fold in the fetus with no further increase after birth. This is associated with an increase in mitochondrial respiratory enzymes and membrane Na/K ATP'ase activity. Catecholamine stimulated, sodium transport dependent respiration increases markedly during this time and accounts for a large part of the fetal increase. Thyroid hormone administration to the newborn rabbit causes an increase in basal but not catecholamine stimulated BAT respiration. We are presently looking at the development of these systems in the fetal sheep animal model.